Diacetals such as dibenzylidene sorbitols and nucleus-substituted dibenzylidene sorbitols are widely used as nucleating agents for polyolefin resins, gelling agents for various fluids, etc. For use as these agents, diacetals need to be dissolved or to be dispersed by molecular order in molten polyolefin resins or fluids.
However, particles of diacetal powders have strong self-agglomeration properties and a high melting point, so that it is not easy to uniformly dissolve or disperse them for industrial purposes. Therefore, some measures must be taken to improve the solubility and dispersibility of diacetals.
A known process for improving the solubility and dispersibility of diacetals comprises treating a diacetal at a temperature equal to or higher than its melting point or melting temperature. However, diacetals, when treated at a high temperature for a long period, undergo heat decomposition or cause coloration, failing to fully exhibit their properties. Thus, the process has problems with functions of diacetals, and is disadvantageous in respect of energy saving.
Japanese Unexamined Patent Publication No. 145431/1994 discloses a process comprising reducing a diacetal to an ultrafine powder to improve the dispersibility and solubility. The ultrafine size powder of solid diacetals, however, deteriorates the working environment since it generates dust which may explode or influence the human body by inhalation thereof. Further, the ultrafine diacetal powder is likely to agglomerate again during storage, and are lower in workabilities such as flow property and transferability (easiness of transferring diacetal powders through piping). Thus, this technique causes industrially serious problems. Moreover, the disclosed technique intends to render the particle size distribution almost monodisperse, thereby necessitating an expensive special milling apparatus.
Further known are processes for increasing the compatibility of a diacetal with polyolefin resins using an organic carboxylic acid in combination with the diacetal (Japanese Unexamined Patent Publication No. 122150/1976, Japanese Examined Patent Publication No. 413/1989 and Japanese Unexamined Patent Publication No. 101131/1985).
The process disclosed in Japanese Unexamined Patent Publication No. 122150/1976 comprises adding dibenzylidene sorbitol and an organic carboxylic acid separately and directly to a polyolefin resin to thereby increase the compatibility of dibenzylidene sorbitol with the resin. The process, however, does not depress the high melting point of the dibenzylidene sorbitol, insufficiently solving the problem of white hard spots formed from undissolved dibenzylidene sorbitol in the resin.
According to the processes of Japanese Examined Patent Publication No. 413/1989 and Japanese Unexamined Patent Publication No. 101131/1985, the surfaces of dibenzylidene sorbitol particles are coated in advance with a higher fatty acid or terephthalic acid, and the coated particles are added to a polyolefin resin. The coated dibenzylidene sorbitol particles have higher compatibility with molten resins than that of uncoated dibenzylidene sorbitol. However, these processes are also unsatisfactory, since the problem of white hard spots of undissolved matter still remains. Thus, the processes do not sufficiently improve the transparency of the resin, and the impaired appearance decreases the commercial value of the product. The same problem arises also when the diacetal particles are surface-coated with a higher fatty acid simply using an organic acid.
Further, a clarifying nucleating agent for polyolefin resins having improved moldability is proposed (Japanese Unexamined Patent Publication No. 245843/1996), which enables molding at a temperature lower than conventional molding temperatures (hereinafter referred to as "low-temperature molding"), the nucleating agent being prepared by mixing an aliphatic carboxylic acid amide and/or an aromatic carboxylic acid amide with a dibenzylidene sorbitol or coating the surfaces of dibenzylidene sorbitol particles with an aliphatic carboxylic acid amide and/or an aromatic carboxylic acid amide. However, said mixing process forms a hard gel when removing the solvent, and thus is difficult to employ for industrial purposes. Said coating process does not exhibit sufficient melting point depressing effect in spite of the large amount of the coating agent, i.e., aliphatic carboxylic acid amide and/or aromatic carboxylic acid amide. Moreover, said amide compounds tends to bleed from polyolefin resin molded articles, and therefore when the coating amount thereof is large, the resulting polyolefin resin molded article prepared from the diacetal coated with said coating agent and a polyolefin resin has problems such as lowered heat-sealing strength. Thus, said process still remains to be improved.